When to use feminine or masculine?

Question

Could you please elaborate more on which condition to use feminine or masculine? For example, why not a lo francés, since it's a style, and why not a ciegos, but a ciegas? Thank you!

Answer

Hola Espanol
These are set expressions that are either used in masculine or in feminine depending on what the precise expression is. In the case of "francés" when we talk about a French omelette we always use it in feminine "tortilla a la francesa", and there is also the expression "despedirse a la francesa" which means to leave a party without saying goodbye. However it wouldn't be unusual to also find (not so much in set expressions) the words "a lo francés" when referring to doing something that resembles somehow the way French people do it, whatever that is. For example, if you organize a party with a French theme, French food, French costumes etc, you could say something like "Estoy preparando una fiesta "a lo francés" (=in a French style, with a French theme...) 
Saludos cordiales
Inma